This page was last updated in 2024-05 and is accurate for router version 0.9.62.

Overview

The I2P Network Protocol (I2NP) is the layer above the I2P transport protocols. It is a router-to-router protocol. It is used for network database lookups and replies, for creating tunnels, and for encrypted router and client data messages. I2NP messages may be sent point-to-point to another router, or sent anonymously through tunnels to that router.

Protocol Versions

All routers must publish their I2NP protocol version in the "router.version" field in the RouterInfo properties. This version field is the API version, indiciating the level of support for various I2NP protocol features, and is not necessarily the actual router version.

If alternative (non-Java) routers wish to publish any version information about the actual router implementation, they must do so in another property. Versions other than those listed below are allowed. Support will be determined through a numeric comparison; for example, 0.9.13 implies support for 0.9.12 features. Note that the "coreVersion" property is no longer published in the router info, and was never used for determination of the I2NP protocol version.

A basic summary of the I2NP protocol versions is as follows. For details, see below.

API Version	Required I2NP Features
0.9.58	Minimum peers will build tunnels through, as of 0.9.62
0.9.55	SSU2 transport support (if published in router info)
0.9.51	Short tunnel build messages for ECIES-X25519 routers Minimum peers will build tunnels through, as of 0.9.58 Minimum floodfill peers will send DSM to, as of 0.9.58
0.9.49	Garlic messages to ECIES-X25519 routers
0.9.48	ECIES-X25519 Routers ECIES-X25519 Build Request/Response records
0.9.46	DatabaseLookup flag bit 4 for AEAD reply
0.9.44	ECIES-X25519 keys in LeaseSet2
0.9.40	MetaLeaseSet may be sent in a DSM
0.9.39	EncryptedLeaseSet may be sent in a DSM RedDSA_SHA512_Ed25519 signature type supported for destinations and leasesets
0.9.38	DSM type bits 3-0 now contain the type; LeaseSet2 may be sent in a DSM
0.9.36	NTCP2 transport support (if published in router info) Minimum peers will build tunnels through, as of 0.9.46
0.9.28	RSA sig types disallowed Minimum floodfill peers will send DSM to, as of 0.9.34
0.9.18	DSM type bits 7-1 ignored
0.9.16	RI key certs / ECDSA and EdDSA sig types Note: RSA sig types also supported as of this version, but currently unused DLM lookup types (DLM flag bits 3-2) Minimum version compatible with vast majority of current network, since routers are now using the EdDSA sig type.
0.9.15	Dest/LS key certs w/ EdDSA Ed25519 sig type (if floodfill)
0.9.12	Dest/LS key certs w/ ECDSA P-256, P-384, and P-521 sig types (if floodfill) Note: RSA sig types also supported as of this version, but currently unused Nonzero expiration allowed in RouterAddress
0.9.7	Encrypted DSM/DSRM replies supported (DLM flag bit 1) (if floodfill)
0.9.6	Nonzero DLM flag bits 7-1 allowed
0.9.3	Requires zero expiration in RouterAddress
0.9	Supports up to 16 leases in a DSM LS store (6 previously)
0.7.12	VTBM and VTBRM message support
0.7.10	Floodfill supports encrypted DSM stores
0.7.9 or lower	All messages and features not listed above
0.6.1.10	TBM and TBRM messages introduced Minimum version compatible with current network

Note that there are also transport-related features and compatibility issues; see the NTCP and SSU transport documentation for details.

Common structures

The following structures are elements of multiple I2NP messages. They are not complete messages.

I2NP message header

Description

Common header to all I2NP messages, which contains important information like a checksum, expiration date, etc.

1 byte [Integer] specifying the type of this message, followed by a 4 byte [Integer] specifying the message-id. After that there is an expiration [Date], followed by a 2 byte [Integer] specifying the length of the message payload, followed by a [Hash], which is truncated to the first byte. After that the actual message data follows.

Standard (16 bytes):

+----+----+----+----+----+----+----+----+
|type|      msg_id       |  expiration
+----+----+----+----+----+----+----+----+
                         |  size   |chks|
+----+----+----+----+----+----+----+----+

Short (SSU, 5 bytes):

+----+----+----+----+----+
|type| short_expiration  |
+----+----+----+----+----+

Short (NTCP2 and SSU2, 9 bytes):

+----+----+----+----+----+----+----+----+
|type|      msg_id       | short_expira-
+----+----+----+----+----+----+----+----+
 tion|
+----+

type :: Integer
        length -> 1 byte
        purpose -> identifies the message type (see table below)

msg_id :: Integer
          length -> 4 bytes
          purpose -> uniquely identifies this message (for some time at least)
                     This is usually a locally-generated random number, but
                     for outgoing tunnel build messages it may be derived from
                     the incoming message. See below.

expiration :: Date
              8 bytes
              date this message will expire

short_expiration :: Integer
                    4 bytes
                    date this message will expire (seconds since the epoch)

size :: Integer
        length -> 2 bytes
        purpose -> length of the payload

chks :: Integer
        length -> 1 byte
        purpose -> checksum of the payload
                   SHA256 hash truncated to the first byte

data ::
        length -> $size bytes
        purpose -> actual message contents

Notes

When transmitted over [SSU], the 16-byte standard header is not used. Only a 1-byte type and a 4-byte expiration in seconds are included. The message id and size are incorporated in the SSU data packet format. The checksum is not required since errors are caught in decryption.
When transmitted over [NTCP2] or [SSU2], the 16-byte standard header is not used. Only a 1-byte type, 4-byte message id, and a 4-byte expiration in seconds are included. The size is incorporated in the NTCP2 and SSU2 data packet formats. The checksum is not required since errors are caught in decryption.
The standard header is also required for I2NP messages contained in other messages and structures (Data, TunnelData, TunnelGateway, and GarlicClove). As of release 0.8.12, to reduce overhead, checksum verification is disabled at some places in the protocol stack. However, for compatibility with older versions, checksum generation is still required. It is a topic for future research to determine points in the protocol stack where the far-end router's version is known and checksum generation can be disabled.
The short expiration is unsigned and will wrap around on Feb. 7, 2106. As of that date, an offset must be added to get the correct time.

BuildRequestRecord

Description

One Record in a set of multiple records to request the creation of one hop in the tunnel. For more details see the tunnel overview [TUNNEL-IMPL] and the ElGamal tunnel creation specification [TUNNEL-CREATION].

For ECIES-X25519 BuildRequestRecords, see [TUNNEL-CREATION-ECIES].

Contents (ElGamal)

[TunnelId] to receive messages on, followed by the [Hash] of our [RouterIdentity]. After that the [TunnelId] and the [Hash] of the next router's [RouterIdentity] follow.

ElGamal and AES encrypted:

+----+----+----+----+----+----+----+----+
| encrypted data...                     |
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+

encrypted_data :: ElGamal and AES encrypted data
                  length -> 528

total length: 528

ElGamal encrypted:

+----+----+----+----+----+----+----+----+
| toPeer                                |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| encrypted data...                     |
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+

toPeer :: First 16 bytes of the SHA-256 Hash of the peer's RouterIdentity
          length -> 16 bytes

encrypted_data :: ElGamal-2048 encrypted data (see notes)
                  length -> 512

total length: 528

Cleartext:

+----+----+----+----+----+----+----+----+
| receive_tunnel    | our_ident         |
+----+----+----+----+                   +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+                   +----+----+----+----+
|                   | next_tunnel       |
+----+----+----+----+----+----+----+----+
| next_ident                            |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| layer_key                             |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| iv_key                                |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| reply_key                             |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| reply_iv                              |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|flag| request_time      | send_msg_id
+----+----+----+----+----+----+----+----+
     |                                  |
+----+                                  +
|         29 bytes padding              |
+                                       +
|                                       |
+                             +----+----+
|                             |
+----+----+----+----+----+----+

receive_tunnel :: TunnelId
                  length -> 4 bytes
                  nonzero

our_ident :: Hash
             length -> 32 bytes

next_tunnel :: TunnelId
               length -> 4 bytes
               nonzero

next_ident :: Hash
              length -> 32 bytes

layer_key :: SessionKey
             length -> 32 bytes

iv_key :: SessionKey
          length -> 32 bytes

reply_key :: SessionKey
             length -> 32 bytes

reply_iv :: data
            length -> 16 bytes

flag :: Integer
        length -> 1 byte

request_time :: Integer
                length -> 4 bytes
                Hours since the epoch, i.e. current time / 3600

send_message_id :: Integer
                   length -> 4 bytes

padding :: Data
           length -> 29 bytes
           source -> random

total length: 222

Notes

In the 512-byte encrypted record, the ElGamal data contains bytes 1-256 and 258-513 of the 514-byte ElGamal encrypted block [CRYPTO-ELG]. The two padding bytes from the block (the zero bytes at locations 0 and 257) are removed.
See the tunnel creation specification [TUNNEL-CREATION] for details on field contents.

BuildResponseRecord

Description

One Record in a set of multiple records with responses to a build request. For more details see the tunnel overview [TUNNEL-IMPL] and the ElGamal tunnel creation specification [TUNNEL-CREATION].

For ECIES-X25519 BuildResponseRecords, see [TUNNEL-CREATION-ECIES].

Contents (ElGamal)

Encrypted:

bytes 0-527 :: AES-encrypted record (note: same size as BuildRequestRecord)

Unencrypted:

+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|                                       |
+   SHA-256 Hash of following bytes     +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| random data...                        |
~                                       ~
|                                       |
+                                  +----+
|                                  | ret|
+----+----+----+----+----+----+----+----+

bytes 0-31   :: SHA-256 Hash of bytes 32-527
bytes 32-526 :: random data
byte  527    :: reply

total length: 528

Notes

The random data field could, in the future, be used to return congestion or peer connectivity information back to the requestor.
See the tunnel creation specification [TUNNEL-CREATION] for details on the reply field.

ShortBuildRequestRecord

For ECIES-X25519 routers only, as of API version 0.9.51. 218 bytes when encrypted. See [TUNNEL-CREATION-ECIES].

ShortBuildResponseRecord

For ECIES-X25519 routers only, as of API version 0.9.51. 218 bytes when encrypted. See [TUNNEL-CREATION-ECIES].

GarlicClove

Unencrypted:

+----+----+----+----+----+----+----+----+
| Delivery Instructions                 |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+
| I2NP Message                          |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+
|    Clove ID       |     Expiration
+----+----+----+----+----+----+----+----+
                    | Certificate  |
+----+----+----+----+----+----+----+

Delivery Instructions :: as defined below
       Length varies but is typically 1, 33, or 37 bytes

I2NP Message :: Any I2NP Message

Clove ID :: 4 byte Integer

Expiration :: Date (8 bytes)

Certificate :: Always NULL in the current implementation (3 bytes total, all zeroes)

Notes

Cloves are never fragmented. When used in a Garlic Clove, the first bit of the Delivery Instructions flag byte specifies encryption. If this bit is 0, the clove is not encrypted. If 1, the clove is encrypted, and a 32 byte Session Key immediately follows the flag byte. Clove encryption is not fully implemented.
See also the garlic routing specification [GARLICSPEC].
Maximum length is a function of the total length of all the cloves and the maximum length of the GarlicMessage.
In the future, the certificate could possibly be used for a HashCash to "pay" for the routing.
The message can be any I2NP message (including a GarlicMessage, although that is not used in practice). The messages used in practice are DataMessage, DeliveryStatusMessage, and DatabaseStoreMessage.
The Clove ID is generally set to a random number on transmit and is checked for duplicates on receive (same message ID space as top-level Message IDs)

Garlic Clove Delivery Instructions

This specification is for Delivery Instructions inside Garlic Cloves only. Note that "Delivery Instructions" are also used inside Tunnel Messages, where the format is significantly different. See the Tunnel Message documentation [TMDI] for details. Do NOT use the following specification for Tunnel Message Delivery Instructions!

+----+----+----+----+----+----+----+----+
|flag|                                  |
+----+                                  +
|                                       |
+       Session Key (optional)          +
|                                       |
+                                       +
|                                       |
+    +----+----+----+----+--------------+
|    |                                  |
+----+                                  +
|                                       |
+         To Hash (optional)            +
|                                       |
+                                       +
|                                       |
+    +----+----+----+----+--------------+
|    |  Tunnel ID (opt)  |  Delay (opt)
+----+----+----+----+----+----+----+----+
     |
+----+

flag ::
       1 byte
       Bit order: 76543210
       bit 7: encrypted? Unimplemented, always 0
                If 1, a 32-byte encryption session key is included
       bits 6-5: delivery type
                0x0 = LOCAL, 0x01 = DESTINATION, 0x02 = ROUTER, 0x03 = TUNNEL
       bit 4: delay included?  Not fully implemented, always 0
                If 1, four delay bytes are included
       bits 3-0: reserved, set to 0 for compatibility with future uses

Session Key ::
       32 bytes
       Optional, present if encrypt flag bit is set.
       Unimplemented, never set, never present.

To Hash ::
       32 bytes
       Optional, present if delivery type is DESTINATION, ROUTER, or TUNNEL
          If DESTINATION, the SHA256 Hash of the destination
          If ROUTER, the SHA256 Hash of the router
          If TUNNEL, the SHA256 Hash of the gateway router

Tunnel ID :: TunnelId
       4 bytes
       Optional, present if delivery type is TUNNEL
       The destination tunnel ID, nonzero

Delay :: Integer
       4 bytes
       Optional, present if delay included flag is set
       Not fully implemented. Specifies the delay in seconds.

Total length: Typical length is:
       1 byte for LOCAL delivery;
       33 bytes for ROUTER / DESTINATION delivery;
       37 bytes for TUNNEL delivery

Messages

Message	Type	Since
DatabaseStore	1
DatabaseLookup	2
DatabaseSearchReply	3
DeliveryStatus	10
Garlic	11
TunnelData	18
TunnelGateway	19
Data	20
TunnelBuild	21
TunnelBuildReply	22
VariableTunnelBuild	23	0.7.12
VariableTunnelBuildReply	24	0.7.12
ShortTunnelBuild	25	0.9.51
OutboundTunnelBuildReply	26	0.9.51
Reserved	0
Reserved for experimental messages	224-254
Reserved for future expansion	255

DatabaseStore

Description

An unsolicited database store, or the response to a successful DatabaseLookup Message

An uncompressed LeaseSet, LeaseSet2, MetaLeaseSet, or EncryptedLeaseset, or a compressed RouterInfo

with reply token:
+----+----+----+----+----+----+----+----+
| SHA256 Hash as key                    |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|type| reply token       | reply_tunnelId
+----+----+----+----+----+----+----+----+
     | SHA256 of the gateway RouterInfo |
+----+                                  +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+    +----+----+----+----+----+----+----+
|    | data ...
+----+-//

with reply token == 0:
+----+----+----+----+----+----+----+----+
| SHA256 Hash as key                    |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|type|         0         | data ...
+----+----+----+----+----+-//

key ::
    32 bytes
    SHA256 hash

type ::
     1 byte
     type identifier
     bit 0:
             0    RouterInfo
             1    LeaseSet or variants listed below
     bits 3-1:
            Through release 0.9.17, must be 0
            As of release 0.9.18, ignored, reserved for future options, set to 0 for compatibility
            As of release 0.9.38, the remainder of the type identifier:
            0: RouterInfo or LeaseSet (types 0 or 1)
            1: LeaseSet2 (type 3)
            2: EncryptedLeaseSet (type 5)
            3: MetaLeaseSet (type 7)
            4-7: Unsupported, invalid
     bits 7-4:
            Through release 0.9.17, must be 0
            As of release 0.9.18, ignored, reserved for future options, set to 0 for compatibility

reply token ::
            4 bytes
            If greater than zero, a DeliveryStatusMessage
            is requested with the Message ID set to the value of the Reply Token.
            A floodfill router is also expected to flood the data to the closest floodfill peers
            if the token is greater than zero.

reply_tunnelId ::
               4 byte TunnelId
               Only included if reply token > 0
               This is the TunnelId of the inbound gateway of the tunnel the response should be sent to
               If $reply_tunnelId is zero, the reply is sent directy to the reply gateway router.

reply gateway ::
              32 bytes
              Hash of the RouterInfo entry to reach the gateway
              Only included if reply token > 0
              If $reply_tunnelId is nonzero, this is the router hash of the inbound gateway
              of the tunnel the response should be sent to.
              If $reply_tunnelId is zero, this is the router hash the response should be sent to.

data ::
     If type == 0, data is a 2-byte Integer specifying the number of bytes that follow,
                   followed by a gzip-compressed RouterInfo. See note below.
     If type == 1, data is an uncompressed LeaseSet.
     If type == 3, data is an uncompressed LeaseSet2.
     If type == 5, data is an uncompressed EncryptedLeaseSet.
     If type == 7, data is an uncompressed MetaLeaseSet.

Notes

For security, the reply fields are ignored if the message is received down a tunnel.
The key is the "real" hash of the RouterIdentity or Destination, NOT the routing key.
Types 3, 5, and 7 are as of release 0.9.38. See proposal 123 for more information. These types should only be sent to routers with release 0.9.38 or higher.
As an optimization to reduce connections, if the type is a LeaseSet, the reply token is included, the reply tunnel ID is nonzero, and the reply gateway/tunnelID pair is found in the LeaseSet as a lease, the recipient may reroute the reply to any other lease in the LeaseSet.
To hide the router OS and implementation, match the Java router implementation of gzip by setting the modification time to 0 and the OS byte to 0xFF, and set XFL to 0x02 (max compression, slowest algorithm). See RFC 1952. First 10 bytes of the compressed router info will be (hex): 1F 8B 08 00 00 00 00 00 02 FF

DatabaseLookup

Description

A request to look up an item in the network database. The response is either a DatabaseStore or a DatabaseSearchReply.

+----+----+----+----+----+----+----+----+
| SHA256 hash as the key to look up     |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| SHA256 hash of the routerInfo         |
+ who is asking or the gateway to       +
| send the reply to                     |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|flag| reply_tunnelId    | size    |    |
+----+----+----+----+----+----+----+    +
| SHA256 of key1 to exclude             |
+                                       +
|                                       |
+                                       +
|                                       |
+                                  +----+
|                                  |    |
+----+----+----+----+----+----+----+    +
| SHA256 of key2 to exclude             |
+                                       +
~                                       ~
+                                  +----+
|                                  |    |
+----+----+----+----+----+----+----+    +
|                                       |
+                                       +
|   Session key if reply encryption     |
+   was requested                       +
|                                       |
+                                  +----+
|                                  |tags|
+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|   Session tags if reply encryption    |
+   was requested                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+

key ::
    32 bytes
    SHA256 hash of the object to lookup

from ::
     32 bytes
     if deliveryFlag == 0, the SHA256 hash of the routerInfo entry this
                           request came from (to which the reply should be
                           sent)
     if deliveryFlag == 1, the SHA256 hash of the reply tunnel gateway (to
                           which the reply should be sent)

flags ::
     1 byte
     bit order: 76543210
     bit 0: deliveryFlag
             0  => send reply directly
             1  => send reply to some tunnel
     bit 1: encryptionFlag
             through release 0.9.5, must be set to 0
             as of release 0.9.6, ignored
             as of release 0.9.7:
             0  => send unencrypted reply
             1  => send AES encrypted reply using enclosed key and tag
     bits 3-2: lookup type flags
             through release 0.9.5, must be set to 00
             as of release 0.9.6, ignored
             as of release 0.9.16:
             00  => normal lookup, return RouterInfo or LeaseSet or
                    DatabaseSearchReplyMessage
                    Not recommended when sending to routers
                    with version 0.9.16 or higher.
             01  => LS lookup, return LeaseSet or
                    DatabaseSearchReplyMessage
                    As of release 0.9.38, may also return a
                    LeaseSet2, MetaLeaseSet, or EncryptedLeaseSet.
             10  => RI lookup, return RouterInfo or
                    DatabaseSearchReplyMessage
             11  => exploration lookup, return DatabaseSearchReplyMessage
                    containing non-floodfill routers only (replaces an
                    excludedPeer of all zeroes)
     bit 4: ECIESFlag
             before release 0.9.46 ignored
             as of release 0.9.46:
             0  => send unencrypted or ElGamal reply
             1  => send ChaCha/Poly encrypted reply using enclosed key
                   (whether tag is enclosed depends on bit 1)
     bits 7-5:
             through release 0.9.5, must be set to 0
             as of release 0.9.6, ignored, set to 0 for compatibility with
             future uses and with older routers

reply_tunnelId ::
               4 byte TunnelID
               only included if deliveryFlag == 1
               tunnelId of the tunnel to send the reply to, nonzero

size ::
     2 byte Integer
     valid range: 0-512
     number of peers to exclude from the DatabaseSearchReplyMessage

excludedPeers ::
              $size SHA256 hashes of 32 bytes each (total $size*32 bytes)
              if the lookup fails, these peers are requested to be excluded
              from the list in the DatabaseSearchReplyMessage.
              if excludedPeers includes a hash of all zeroes, the request is
              exploratory, and the DatabaseSearchReplyMessage is requested
              to list non-floodfill routers only.

reply_key ::
     32 byte key
     see below

tags ::
     1 byte Integer
     valid range: 1-32 (typically 1)
     the number of reply tags that follow
     see below

reply_tags ::
     one or more 8 or 32 byte session tags (typically one)
     see below

Reply Encryption

Flag bit 4 is used in combination with bit 1 to determine the reply encryption mode. Flag bit 4 must only be set when sending to routers with version 0.9.46 or higher. See proposals 154 and 156 for details.

In the table below, "DH n/a" means that the reply is not encrypted. "DH no" means that the reply keys are included in the request. "DH yes" means that the reply keys are derived from the DH operation.

Flag bits 4,1	From	To Router	Reply	DH?	notes
0 0	Any	Any	no enc	n/a	no encryption
0 1	ElG	ElG	AES	no	As of 0.9.7
1 0	ECIES	ElG	AEAD	no	As of 0.9.46
1 0	ECIES	ECIES	AEAD	no	As of 0.9.49
1 1	ElG	ECIES	AES	yes	TBD
1 1	ECIES	ECIES	AEAD	yes	TBD

No Encryption

reply_key, tags, and reply_tags are not present.

ElG to ElG

Supported as of 0.9.7. ElG destination sends a lookup to a ElG router.

Requester key generation:

reply_key :: CSRNG(32) 32 bytes random data
reply_tags :: Each is CSRNG(32) 32 bytes random data

Message format:

reply_key ::
     32 byte SessionKey big-endian
     only included if encryptionFlag == 1 AND ECIESFlag == 0, only as of release 0.9.7

tags ::
     1 byte Integer
     valid range: 1-32 (typically 1)
     the number of reply tags that follow
     only included if encryptionFlag == 1 AND ECIESFlag == 0, only as of release 0.9.7

reply_tags ::
     one or more 32 byte SessionTags (typically one)
     only included if encryptionFlag == 1 AND ECIESFlag == 0, only as of release 0.9.7

ECIES to ElG

Supported as of 0.9.46. ECIES destination sends a lookup to a ElG router. The reply_key and reply_tags fields are redefined for an ECIES-encrypted reply.

Requester key generation:

reply_key :: CSRNG(32) 32 bytes random data
reply_tags :: Each is CSRNG(8) 8 bytes random data

Message format: Redefine reply_key and reply_tags fields as follows:

reply_key ::
     32 byte ECIES SessionKey big-endian
     only included if encryptionFlag == 0 AND ECIESFlag == 1, only as of release 0.9.46

tags ::
     1 byte Integer
     required value: 1
     the number of reply tags that follow
     only included if encryptionFlag == 0 AND ECIESFlag == 1, only as of release 0.9.46

reply_tags ::
     an 8 byte ECIES SessionTag
     only included if encryptionFlag == 0 AND ECIESFlag == 1, only as of release 0.9.46

The reply is an ECIES Existing Session message, as defined in [ECIES].

Reply format

This is the existing session message, same as in [ECIES], copied below for reference.

+----+----+----+----+----+----+----+----+
|       Session Tag                     |
+----+----+----+----+----+----+----+----+
|                                       |
+            Payload Section            +
|       ChaCha20 encrypted data         |
~                                       ~
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|  Poly1305 Message Authentication Code |
+              (MAC)                    +
|             16 bytes                  |
+----+----+----+----+----+----+----+----+

Session Tag :: 8 bytes, cleartext

Payload data :: remaining data minus 16 bytes

MAC :: Poly1305 message authentication code, 16 bytes

AEAD parameters:

tag :: 8 byte reply_tag

k :: 32 byte session key
   The reply_key.

n :: 0

ad :: The 8 byte reply_tag

payload :: Plaintext data, the DSM or DSRM.

ciphertext = ENCRYPT(k, n, payload, ad)

ECIES to ECIES (0.9.49)

ECIES destination or router sends a lookup to a ECIES router. Supported as of 0.9.49.

ECIES routers were introduced in 0.9.48, see [Prop156]. ECIES destinations and routers may use the same format as in the "ECIES to ElG" section above, with reply keys included in the request. The lookup message encryption is specified in [ECIES-ROUTERS]. The requester is anonymous.

ECIES to ECIES (future)

This option is not yet fully defined. See [Prop156].

Notes

Prior to 0.9.16, the key may be for a RouterInfo or LeaseSet, as they are in the same key space, and there was no flag to request only a particular type of data.
Encryption flag, reply key, and reply tags as of release 0.9.7.
Encrypted replies are only useful when the response is through a tunnel.
The number of included tags could be greater than one if alternative DHT lookup strategies (for example, recursive lookups) are implemented.
The lookup key and exclude keys are the "real" hashes, NOT routing keys.
Types 3, 5, and 7 may be returned as of release 0.9.38. See proposal 123 for more information.

DatabaseSearchReply

Description

The response to a failed DatabaseLookup Message

A list of router hashes closest to the requested key

+----+----+----+----+----+----+----+----+
| SHA256 hash as query key              |
+                                       +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
| num| peer_hashes                      |
+----+                                  +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+    +----+----+----+----+----+----+----+
|    | from                             |
+----+                                  +
|                                       |
+                                       +
|                                       |
+                                       +
|                                       |
+    +----+----+----+----+----+----+----+
|    |
+----+

key ::
    32 bytes
    SHA256 of the object being searched

num ::
    1 byte Integer
    number of peer hashes that follow, 0-255

peer_hashes ::
          $num SHA256 hashes of 32 bytes each (total $num*32 bytes)
          SHA256 of the RouterIdentity that the other router thinks is close
          to the key

from ::
     32 bytes
     SHA256 of the RouterInfo of the router this reply was sent from

Notes

The 'from' hash is unauthenticated and cannot be trusted.
The returned peer hashes are not necessarily closer to the key than the router being queried. For replies to regular lookups, this facilitates discovery of new floodfills and "backwards" searching (further-from-the-key) for robustness.
The key for an exploration lookup is usually generated randomly. Therefore, the response's non-floodfill peer_hashes may be selected using an optimized algorithm, such as providing peers that are close to the key but not necessarily the closest in the entire local network database, to avoid an inefficient sort or search of the entire local database. Other strategies such as caching may also be appropriate. This is implementation-dependent.
Typical number of hashes returned: 3
Recommended maximum number of hashes to return: 16
The lookup key, peer hashes, and from hash are "real" hashes, NOT routing keys.

DeliveryStatus

Description

A simple message acknowledgment. Generally created by the message originator, and wrapped in a Garlic Message with the message itself, to be returned by the destination.

The ID of the delivered message, and the creation or arrival time.

+----+----+----+----+----+----+----+----+----+----+----+----+
| msg_id            |           time_stamp                  |
+----+----+----+----+----+----+----+----+----+----+----+----+

msg_id :: Integer
       4 bytes
       unique ID of the message we deliver the DeliveryStatus for (see
       I2NPMessageHeader for details)

time_stamp :: Date
             8 bytes
             time the message was successfully created or delivered

Notes

It appears that the time stamp is always set by the creator to the current time. However there are several uses of this in the code, and more may be added in the future.
This message is also used as a session established confirmation in SSU [SSU-ED]. In this case, the message ID is set to a random number, and the "arrival time" is set to the current network-wide ID, which is 2 (i.e. 0x0000000000000002).

Garlic

Description

Used to wrap multiple encrypted I2NP Messages

When decrypted, a series of Garlic Cloves and additional data, also known as a Clove Set.

Encrypted:

+----+----+----+----+----+----+----+----+
|      length       | data              |
+----+----+----+----+                   +
|                                       |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+

length ::
       4 byte Integer
       number of bytes that follow 0 - 64 KB

data ::
     $length bytes
     ElGamal encrypted data

Decrypted data, also known as a Clove Set:

+----+----+----+----+----+----+----+----+
| num|  clove 1                         |
+----+                                  +
|                                       |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+
|         clove 2 ...                   |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+
| Certificate  |   Message_ID      |
+----+----+----+----+----+----+----+----+
          Expiration               |
+----+----+----+----+----+----+----+

num ::
     1 byte Integer number of GarlicCloves to follow

clove ::  a GarlicClove

Certificate :: always NULL in the current implementation (3 bytes total, all zeroes)

Message_ID :: 4 byte Integer

Expiration :: Date (8 bytes)

Notes

When unencrypted, data contains one or more Garlic Cloves.
The AES encrypted block is padded to a minimum of 128 bytes; with the 32-byte Session Tag the minimum size of the encrypted message is 160 bytes; with the 4 length bytes the minimum size of the Garlic Message is 164 bytes.
Actual max length is less than 64 KB; see [I2NP].
See also the ElGamal/AES specification [ELG-AES].
See also the garlic routing specification [GARLIC].
The 128 byte minimum size of the AES encrypted block is not currently configurable, however the minimum size of a DataMessage in a GarlicClove in a GarlicMessage, with overhead, is 128 bytes anyway. A configurable option to increase the minimum size may be added in the future.
The message ID is generally set to a random number on transmit and appears to be ignored on receive.
In the future, the certificate could possibly be used for a HashCash to "pay" for the routing.

TunnelData

Description

A message sent from a tunnel's gateway or participant to the next participant or endpoint. The data is of fixed length, containing I2NP messages that are fragmented, batched, padded, and encrypted.

+----+----+----+----+----+----+----+----+
|     tunnnelID     | data              |
+----+----+----+----+                   |
|                                       |
~                                       ~
~                                       ~
|                                       |
+                   +----+----+----+----+
|                   |
+----+----+----+----+

tunnelId ::
         4 byte TunnelId
         identifies the tunnel this message is directed at
         nonzero

data ::
     1024 bytes
     payload data.. fixed to 1024 bytes

Notes

The I2NP message ID for this message is set to a new random number at each hop.
See also the Tunnel Message Specification [TUNNEL-MSG]

TunnelGateway

Description

Wraps another I2NP message to be sent into a tunnel at the tunnel's inbound gateway.

+----+----+----+----+----+----+----+-//
| tunnelId          | length  | data...
+----+----+----+----+----+----+----+-//

tunnelId ::
         4 byte TunnelId
         identifies the tunnel this message is directed at
         nonzero

length ::
       2 byte Integer
       length of the payload

data ::
     $length bytes
     actual payload of this message

Notes

The payload is an I2NP message with a standard 16-byte header.

Data

Description

Used by Garlic Messages and Garlic Cloves to wrap arbitrary data.

A length Integer, followed by opaque data.

+----+----+----+----+----+-//-+
| length            | data... |
+----+----+----+----+----+-//-+

length ::
       4 bytes
       length of the payload

data ::
     $length bytes
     actual payload of this message

Notes

This message contains no routing information and will never be sent "unwrapped". It is only used inside Garlic messages.

TunnelBuild

+----+----+----+----+----+----+----+----+
| Record 0 ...                          |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+
| Record 1 ...                          |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+
| Record 7 ...                          |
~                                       ~
~                                       ~
|                                       |
+----+----+----+----+----+----+----+----+

Just 8 BuildRequestRecords attached together
record size: 528 bytes
total size: 8*528 = 4224 bytes

Notes

As of 0.9.48, may also contain ECIES-X25519 BuildRequestRecords, see [TUNNEL-CREATION-ECIES].
See also the tunnel creation specification [TUNNEL-CREATION].
The I2NP message ID for this message must be set according to the tunnel creation specification.
While this message is rarely seen in today's network, having been replaced by the VariableTunnelBuild message, it may still be used for very long tunnels, and has not been deprecated. Routers must implement.

TunnelBuildReply

Same format as TunnelBuildMessage, with BuildResponseRecords

Notes

As of 0.9.48, may also contain ECIES-X25519 BuildResponseRecords, see [TUNNEL-CREATION-ECIES].
See also the tunnel creation specification [TUNNEL-CREATION].
The I2NP message ID for this message must be set according to the tunnel creation specification.
While this message is rarely seen in today's network, having been replaced by the VariableTunnelBuildReply message, it may still be used for very long tunnels, and has not been deprecated. Routers must implement.

VariableTunnelBuild

+----+----+----+----+----+----+----+----+
| num| BuildRequestRecords...
+----+----+----+----+----+----+----+----+

Same format as TunnelBuildMessage, except for the addition of a $num field
in front and $num number of BuildRequestRecords instead of 8

num ::
       1 byte Integer
       Valid values: 1-8

record size: 528 bytes
total size: 1+$num*528

Notes

As of 0.9.48, may also contain ECIES-X25519 BuildRequestRecords, see [TUNNEL-CREATION-ECIES].
This message was introduced in router version 0.7.12, and may not be sent to tunnel participants earlier than that version.
See also the tunnel creation specification [TUNNEL-CREATION].
The I2NP message ID for this message must be set according to the tunnel creation specification.
Typical number of records in today's network is 4, for a total size of 2113.

VariableTunnelBuildReply

+----+----+----+----+----+----+----+----+
| num| BuildResponseRecords...
+----+----+----+----+----+----+----+----+

Same format as VariableTunnelBuildMessage, with BuildResponseRecords.

Notes

As of 0.9.48, may also contain ECIES-X25519 BuildResponseRecords, see [TUNNEL-CREATION-ECIES].
This message was introduced in router version 0.7.12, and may not be sent to tunnel participants earlier than that version.
See also the tunnel creation specification [TUNNEL-CREATION].
The I2NP message ID for this message must be set according to the tunnel creation specification.
Typical number of records in today's network is 4, for a total size of 2113.

ShortTunnelBuild

Description

As of API version 0.9.51, for ECIES-X25519 routers only.

+----+----+----+----+----+----+----+----+
| num| ShortBuildRequestRecords...
+----+----+----+----+----+----+----+----+

Same format as VariableTunnelBuildMessage,
except that the record size is 218 bytes instead of 528

num ::
       1 byte Integer
       Valid values: 1-8

record size: 218 bytes
total size: 1+$num*218

Notes

As of 0.9.51. See [TUNNEL-CREATION-ECIES].
This message was introduced in router version 0.9.51, and may not be sent to tunnel participants earlier than that version.
Typical number of records in today's network is 4, for a total size of 873.

OutboundTunnelBuildReply

Description

Sent from the outbound gateway of a new tunnel to the originator. As of API version 0.9.51, for ECIES-X25519 routers only.

+----+----+----+----+----+----+----+----+
| num| ShortBuildResponseRecords...
+----+----+----+----+----+----+----+----+

Same format as ShortTunnelBuildMessage, with ShortBuildResponseRecords.